The present invention relates to a process for applying nitride layers to parts composed of titanium and titanium alloys by thermochemical treatment of the parts with ammonia or ammonia-containing gas mixtures under pressures higher than atmospheric pressure and at temperatures above 500.degree. C.
Titanium has some advantages over steel as a construction material, namely its low specific weight, its corrosion-resistance, and its high strength. On the other hand, its hardness is relatively low, which necessitates a surface treatment to increase wear-resistance. This surface treatment generally involves generating layers of titanium carbide or titanium nitride. Processes known hitherto for nitriding parts composed of titanium and titanium alloys involve the use of high-energy gases or electromagnetic fields. These processes are very expensive and are only applicable with parts having a simple geometry.
For example, in German Patent 17 96 212 (which is entirely incorporated herein by reference), the surface hardening of titanium by the formation of nitride layers in an ammonia atmosphere at relatively high temperatures and normal pressure is mentioned. Although the process described in this patent is intended to produce relatively thick, hard layers, this process has no practical application, since the high temperature induces detrimental structural modifications of the core of the component. The component becomes brittle.
Brunner, European Patent Application No. 0,105,835 (which is entirely incorporated herein by reference) describes a process for manufacturing nitride layers on components composed of titanium and titanium alloys by exposing the components in an autoclave to pressures of at least 10 MPa and temperatures of at least 200.degree. C. in an ammonia atmosphere. The actual examples in Brunner use pressures much higher than 10 MPa. For the process of Brunner, the ammonia must be very pure. Preferably, the nitriding takes place at 90 to 130 MPa and temperatures of 930.degree. C. to 1000.degree. C., as shown in Brunner's examples. This process has the disadvantage that it is very expensive due to the use of autoclaves and because very pure ammonia must be used. Furthermore, layers 20 .mu.m thick can be obtained only in treatment periods of three or more hours. Additionally, because of these high pressures, an autoclave is used by Brunner. Because of limitations on the size of autoclaves, large parts and large batches of parts cannot be treated. This makes the process of Brunner economically unfeasible.